With the development of High Density Wavelength Division Multiplexing (HDWDM) applications, for example as used in communications conducted via optical fiber transmission mediums, the need for quality optical spectrum receivers and analyzers has become acute. In particular, there is a demand for optical spectrum analyzer (OSA) instruments that are robust, compact and yet display a sufficient optical rejection ratio (ORR) close in to a spectral feature (e.g, laser carrier signal) to be measured. This is particularly so for portable instruments that are to be used in the field.
As described in the text book “Optical Spectrum Analysis”, authored by Joachim Vobis and Dennis Derickson, the three main optical spectrum resolving technologies that have been used in optical spectrum analyzers (OSAs) are Michelson interferometers, tunable Fabry-Perot optical filters and diffraction gratings.
OSAs employing Michelson interferometers (coupled with Fourier transform analysis) provide high wavelength precision and good spectral resolution, but the robustness and effective ORR limitations of such instruments continue to present problems. For example, the maximum effective ORR presently achievable is in the order of 35 dB (even far from the spectral feature), which is far less than what is desired.
OSAs using tunable Fabry-Perot filters are usually compact and rugged in nature. The various designs require trade-offs, however, between resolution, free spectral range and ORR. ORR can be improved by using multi-cavity filters, or cascading filters or by the multiple passes through the filter. However, when manufacturing multi-cavity filters using coating techniques, it is quite difficult to match the cavities. Three-cavity filters are presently available with a spectral resolution (FWHM) of 0.5 to 1.0 nm. The paper entitled “Multiple Angle-Tuned Etalon Filters for Optical Channel Selection in Wavelength Division Multiplexed and Optical Frequency Division Multiplexed Direct Detection Transmission Systems” by Anatoly Frenkel and Chinlon Lin—Opt. Lett. Vol. 13, pp 684–686, 1988 describes instruments with cascaded identical filters and cascaded nonidentical filters. Although individual angle-tuned filter elements exhibit properties apparently suitable for optical wavelength discrimination, generally, to cascade two or more filters is complicated because of inter-cavity interference problems. Also, when non-identical filters are cascaded, tuning synchronization may present problems.
Multi-pass filters are disclosed in the paper entitled “The Design and Use of a Stabilised Multi-passed Interferometer of High Contrast Ratio” by J. R. Sandercock in the published Proceedings of the Second International Conference of Light Scattering in Solids, Flammarion, Paris, pp. 9 to 12, 1971, and U.S. Pat. No. 3,729,261 which issued on Apr. 24, 1973 naming John R. Sandercock as an inventor. Sandercock describes a 5-pass filter configuration which uses a pair of corner cube retroreflectors. As corner cube retroreflectors cannot maintain the polarization states between the input and output beams, the filter used must be polarization-insensitive in both insertion loss and wavelength splitting. Since angle-tuned filter elements are polarization-sensitive, they cannot be used.
A wide spectrum of optical energy, from infrared through the visible spectrum, is commonly used as a means of conveying information via various optical fiber transmission media. In the telecommunications industry optical semiconductor lasers are typically used as light sources. Although the beam emitted by an optical laser tends to be of a fixed linear polarization, the emitted beam is typically received after having traversed various optical fiber conduits, so the received beam's state of polarization is not accurately predictable. The received beam's state of polarization may vary in an unpredictable manner over a period of time. Because angle-tuned filters are inherently polarization-sensitive, they are not practical for use in detecting the spectral characteristics of such a beam of unknown or unpredictable polarization. In addition, angle-tuned filter elements available commercially at present have a wavelength range of no more than about 100 nm, so the operational bandwidth of an optical spectrum analyzer using an angle-tuned filter element would be limited.
A diffraction grating, in combination with input and output slits, may be used as a tunable optical filter, and the optical spectral resolution improved by reducing the size of the input and output slits. An OSA using such a device could be expected to have a larger (e.g. of 500 nm or more) than an OSA using a tunable Fabry-Perot filter. However, the state of polarization (SOP) of incident light must be linear and parallel to a dispersion plane of the grating in order to be reflected or transmitted efficiently. Diffraction gratings are inherently polarization-sensitive, except perhaps for one particular wavelength.
Diffraction grating technology is the most widely used for OSAs in fiber testing equipment As was the case for the Michelson interferometer technology, many efforts have been made to improve the ORR close in to a spectral feature (e.g., ±0.4 nm), as well as the robustness of such devices. For example, approaches to improve the ORR include the use of double monochromators or the use of a double pass/double filtering process to make the lines sharper. The disadvantage of this approach is that it requires an intermediate spatial filter (i.e. a slit) and it tends to be bulkier, less robust and more expensive.
U.S. Pat. No. 5,886,785 (Lefevre et al.) discloses an OSA in which an input light beam is separated into two linearly-polarized secondary light beams, one of which is then rotated by a halfwave (λ/2) plate so that their states of polarization are parallel. The parallel beams are reflected by a pair of right-angled dihedral reflectors so that they are diffracted four times by the same diffraction grating before being recombined at the output of the OSA. Because the two light beams follow the same path (in opposite directions) between the reflectors and the grating, any polarization-dependent losses affect them both equally.
There are several disadvantages to the OSA disclosed by Lefevre et al. Firstly, the fact that the two light beams are recombined at the output is not conducive to the use of such an OSA for some applications where it is desirable to know the power of each of the two orthogonal components of the input light beam. Secondly, the waveplate used in their invention introduces a half-wave retardance at only one wavelength. Consequently, for other wavelengths, the resulting SOP exiting the waveplate will deviate from the desired linear SOP and lead to additional loss in the OSA. If used over a relatively limited wavelength range (e.g. 1480 nm–1620 nm), this loss is not major and can be compensated in the firmware, but if used over a much wider range typical of Coarse Wavelength Division Multiplexing (CWDM) (e.g. 1280 nm–1620 nm), this additional loss can significantly degrade the sensitivity of the OSA. Thirdly, the preferred embodiment of Lefevre et al.'s OSA uses the endface of the same fiber as a common input and output slit, and hence the design requires a circulator, 50/50 coupler or other separation means to separate the returning light for subsequent detection. Any polarisation dependent loss (PDL) in the separation means will limit the accuracy of intensity measurements made by the OSA Finally, the use of the same fiber for both the input and output renders the OSA very susceptible to any residual backreflection from the various component parts. For instance, a small reflection from the initial collimating lens after the fiber endface will reflect back into the detection means. If the signal under test comprises, say, 40 DWDM channels, this could introduce a significant amount of background noise to the desired measurement.